This invention is generally directed to processes for the preparation of photogenerating compositions, and more specifically, the present invention is directed to processes for the sublimation preparation and purification of photogenerating pigments, such as benzimidazole perylene in high purity, for example from about 95 to about 99.9 percent pure, and with excellent, or improved photoelectrical characteristics, such as low dark decay, and acceptable photosensitivity by utilizing pelletized crude starting components. In one embodiment of the present invention, there are provided economical processes for the preparation of high purity photogenerating pigments by a fractionation sublimation method which involves the stepwise sublimation of a pelletized crude pigment, such as benzimidazole perylene from an evaporation source crucible. The fractionation sublimation may comprise two or more sublimation processes. In one embodiment of a two-step fractionation sublimation, the sublimation temperature in the initial step is slightly above, for example from about 300.degree. to about 550.degree. C., and more specifically, for benzimidazole perylene and for phthalocyanines it is about 500.degree. to about 530.degree. C., and for dibromoanthanthrone it is from about 300.degree. to about 350.degree. C., the subliming temperature of the pigment such that an effective amount, for example, from between about 5 to about 20 weight percent of the sublimate is deposited by the condensation of the vapor of the sublimed material onto a collector substrate, or sheet of, for example, glass, quartz, metals such as stainless steel, and aluminum. Subsequently, the sublimation temperature in the second step is increased by 10.degree. to 100.degree. C. for an effective period of time, for example from about 1 hour to 3 hours for each kilogram of crude pigment, for example, until from between about 50 to about 80 weight percent of the resulting high photoelectrical sublimate photogenerating pigment is collected on a second substrate. Optionally, the fractionation sublimation processes of the present invention may include multisublimation steps, that is, for example, more than two and up to 10 in embodiments. The use of pelletized crude rather than the powder assynthesized crude can provide improvement in the purity and electrical properties of the final sublimates. Sublimable photogenerating pigments, such as phthalocyanines, perinones, perylenes, polycyclic aromatic coumpounds, pyrrolopyrroles, polycyclic quinones, cyanines and the like, can be prepared by the processes as illustrated herein in embodiments. In particular, the conditions for purifying benzimidazole perylene in the twostep sublimation are chosen such that the temperature range in the first step is controlled at between 500.degree. to about 530.degree. C. and the temperature range in the second step is maintained between 540.degree. to 600.degree. C.
The electrical performance of photoresponsive members used in electrophotographic applications can depend on the purity of the photogenerating pigments. Generally, the photosensitivity, cyclic stability, and charging properties of photoresponsive members can be severely degraded by the presence of certain impurities in the photogenerating pigments. Unfortunately, many of the known photogenerating pigments are not easily purified by chemical methods because, for example, of their extremely poor solubilities in organic solvents. For example, the prior art discloses the selection of strong acids and strong bases in an attempt to dissolve these pigments for purification purposes. However, detrimental byproducts and additional impurities are produced in these chemical methods causing significant degradation in electrical properties of the final photogenerating pigment materials. Filtration of fine pigment particles which are reprecipitated from the acid or base solutions in the aforementioned purification process is usually a time-consuming process extending over about two weeks in embodiments. Furthermore, the chemical methods can generate large quantities of waste, for example when 1,152 killigrams of sulfuric acid, ammonium hydroxide and solvents such as dimethylformamide are used in purifying 3.5 killigrams of VOPc by acid pasting method, reference U.S. Pat. No. 4,771,133, the weight ratio of waste chemicals to pigment is high, that is about 300 times quantities of waste materials such as acids, bases, byproducts from the acid pasting purification method have to be properly disposed of to minimize environmental pollution. Risks of causing poor pigment quality, waste chemical disposal, and additional costs encountered in the prior art chemical purification approach are serious disadvantages that are avoided or eliminated with the processes of the present invention.
Photoresponsive members provided with a photogenerating layer comprised of a vacuum deposition of organic pigment are disclosed in U.S. Pat. Nos. 4,587,189; 4,578,334 and 4,555,463. The preparation of the generator layer requires large vacuum coating equipment wherein it is difficult to control the uniformity in thickness of thin generator layer over extended area and length of the conductive substrate. Moreover, to form a complete photoresponsive member, the generating layer is to be overcoated with charge transport layer by a different coating technique, namely, solution coating. As a result, two separate coating facilities and processes are involved which can significantly increase the cost of production. In practice, a single coating technique will be more desirable from an economic and manufacturing standpoint.
The present invention involves in embodiments purifying sublimable organic photogenerating pigments by a fractionation sublimation process and incorporating the resulting sublimed pigment into photoresponsive members. The preparation of photoresponsive members is preferably accomplished by solution coating of both photogenerating and transport layers. The solution coating may be dip, spray, slot, or web coating methods. The sublimation process may also lends itself as a means to purify a crude pigment which can then be used in the vacuum coating of a thin photogenerating layer. The vacuum coated photogenerating layer would have significantly reduced coating defects since the sublimation of pigment in accordance with the processes of the present invention prior to the vacuum coating can remove some volatile impurities which would have been otherwise been incorporated into the photogenerating layer.
Certain sublimation process such as train sublimation are illustrated in U.S. Pat. Nos. 4,952,471 and 4,952,472 and by H. J. Wagner in J. Materials Science, 17, 2781 (1982). The train sublimation process involves passing a carrier gas over a crude material in a glass tube situated in a specially designed oven which had a temperature gradient. By adjusting the flow of gas and controlling the temperature profile of the oven, it is possible to effect the sublimation of pigment to form vapor at the high temperature zone, which vapor was then carried to downstream and condensed to form solid deposits at the lower temperature zones. The desired sublimed pigment was deposited at the medium temperature zone and the more volatile impurities at the low temperature zone. A separation of impurities from the sublimed pigment was hence achieved. The train sublimation process was generally not efficient as desired as the presence of carrier gas (pressure between 1 and 10 Torr) greatly decreased, for example by a factor of ten, the sublimation rate of material and hence prolonged the duration of sublimation. The conventional operation spanned over 12 hours for even a small amount, about 10 grams, of starting material used in the sublimation. In addition, the furnace had to be sufficiently long and its temperature profile properly controlled in order to achieve a good spatial separation of purified sublimed material from the crude and more volatile impurities. These requirements can impose great complexities in the design of large production equipment and cause complex costly operations. The train sublimation, though useful in obtaining small amounts of purified materials, is not believed to be economically convertable to large scale productions.
Also, there is disclosed in U.S. Pat. No. 4,431,722 a vacuum sublimation process for a class of polycyclic quinone pigments such as anthanthrone, dibenzpyrenequinone, and pyranthrone derivatives. These pigments have low subliming temperatures of 350.degree. C. This patent does not disclose removing volatile impurities which could contaminate the final sublimed pigment. Volatile impurities could be present in the crude material or produced as decomposition products during the initial heating of the crude material. For other pigments having a higher subliming temperature greater than 450.degree. C., such as perylenes, and phthalocyanines, the higher temperature heating in the initial stage of sublimation can cause the formation of significantly large amount of volatile decomposition impurities. As a result, these volatile impurities cannot be separated from the deposited sublimed material and the benefits of sublimation process were not fully realized. The shortcomings of the process of the '722 patent were reflected in the results of simple sublimation of various pigments which were summarized in Table 1 of this patent. The observed changes in electrical properties shown in Table 1 amounted to some improvement, about 25 percent, in photosensitivities as indicated by the reduction in E.sub.1/2 values. However, worsening in the charging property as seen in the decreasing V.sub.o occured for all pigments attempted in the sublimation trials. The decrease reached up to 139 volts in one case which suggests a severe contamination of sublimed pigment by impurities occuring in the simple sublimation process. Also, this process failed to recognize the need of preventing the ejection of crude materials, especially those in the form of light and fluffy powder, from the crucible onto the sublimed material which would become contaminated.
With the fractionation sublimation process of the present invention there can be enabled in embodiments, for example, the separation of volatile component, impurities such as residual (unreacted) phthalonitrile in the crude phthalocyanines, residual perylene tetracarboxylic dianhydride in the crude perylene pigments, into the first fraction of sublimate and these impurities will not then cause contamination into the second or subsequent fractions of the sublimate. The aforementioned volatile impurities can be those originally present in the crude material or produced during initial heating of crude material. Also, the use of a pelletized crude material eliminates contamination problems posed by prior art fluffy powder crude materials. Pellets are capable of holding the powder together during handling and sublimation, whereas fluffy powder crude as well as residual ashes formed tend to eject from the evaporation crucible during sublimation and become incorporated into the sublimate. The impurities from crude powder and residual ashes are detrimental to the electrical properties of sublimate collected. In the '722 patent no fractionation is involved, and the separation of volatile impurities from the final sublimate is not accomplished, thus volatile impurities are incorporated into the sublimed product material.
Although the sublimation processes for purifying organic pigments have been described in the prior art, there remains a need for developing a sublimation process which is more capable of producing highly purified sublimed materials in a cost effective and controllable manner, and yet does not have many of the aforementioned shortcomings. The invention of the present application is directed to an improved fractionation sublimation process wherein, for example, high purity organic photogenerating pigment suitable for electrophotographic imaging applications can be obtained. The process, for example, involves fractionation sublimations using a pelletized crude starting material. In particular, the process of the present invention in embodiments can remove unreacted perylene tetracarboxylic dianhydride in the perylene crude and phthalonitrile in the phthalocyanine crude, volatile impurities from contaminating the sublimed materials. The volatile impurities may be those already present as byproducts in the crude material which are formed during the chemical synthesis of pigment. They could also be produced as decomposition byproducts during the initial heating process of sublimation. Therefore, the fractionation process of the present invention can allow for the control of the quality of the sublimed materials by separating these volatile impurities from the desired fractions. Moreover, the process of the present invention involves the use of pelletized crude pigment in a sublimation method which virtually eliminates the direct contamination of final sublimed product by the crude material. During sublimation, the pelletized crude material can hold the powder in compact form and prevent the ejection of crude material from the crucible onto the collector where it can be incorporated into the sublimed material. However, the uncompressed, light and fluffy crude powder can be easily projected onto the collector, especially at high sublimation rate. Furthermore, the powder of photogenerating pigments is usually insulating in nature and have a tendency to undergo triboelectric charging due to friction and form a floating cloud during handling. These floating particles of crude material can rise to deposit onto the sublimed material collected in the sublimation equipment, and a severe contamination of the sublimed pigment by impurities can greatly degrade the photoresponsive performance of the final product. Thus, the powder crude selected for the processes of the present invention can be compressed into compact pellets which avoid the disadvantages associated with light and fluffy components, which are not free floating clouds.
Purified, sublimed organic pigments prepared by this invention are useful for the preparation of electrophotographic imaging members which exhibit improved electrical properties such as high charge acceptance, stable charging and high photosensitivity. The imaging members generally comprise a photosensitive layer composed of a pigment-containing layer prepared either by solution coating a dispersion of sublimed material in polymeric slurry or solvent, or by vacuum coating of solid sublimed material. In one device configuration, the imaging members contain separate photogeneration and transport layers coated on suitable conductive substrate. In another device configuration, the photogeneration and charge transport functions occur within a single composite layer. Examples of both types of device configurations are described in U.S. Pat. Nos. 4,265,990; 4,514,482; 4,937,164, and related patents, the disclosures of which are totally incorporated herein by reference.
Documents illustrating organic electrophotographic photoconductor elements with azo, bisazo, and related compounds include U.S. Pat. Nos. 4,390,611, 4,551,404, 4,596,754, Japanese Patent 60-64354, U.S. Pat. Nos. 4,400,455, 4,390,608, 4,327,168, 4,299,896, 4,314,015, 4,486,522, 4,486,519, 4,555,667, 4,440,845, 4,486,800, 4,309,611, 4,418,133, 4,293,628, 4,427,753, 4,495,264, 4,359,513, 3,898,084, 4,830,944, 4,820,602, and Japanese Patent Publication 60-111247.